Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of the fuel (such as hydrogen) and the oxidant (such as air or oxygen) to generate electricity. Water is a byproduct of the electrochemical reaction. The PEM is a solid polymer electrolyte that facilitates transfer of protons from an anode electrode to a cathode electrode in each individual fuel cell of a stack of fuel cells normally deployed in a fuel cell power system.
In the typical fuel cell stack, the individual fuel cells have fuel cell plates with channels, through which various reactants and cooling fluids flow. Fuel cell plates may be unipolar, for example. A bipolar plate may be formed by combining a plurality of unipolar plates. Movement of water from the channels to an outlet header and through a tunnel region formed by the fuel cell plates is caused by the flow of the reactants through the fuel cell assembly. Boundary layer shear forces and the reactant pressure aid in transporting the water through the channels and the tunnel region until the water exits the fuel cell through the outlet header.
A membrane-electrolyte-assembly (MEA) is disposed between successive plates to facilitate the electrochemical reaction. The MEA includes the anode electrode, the cathode electrode, and an electrolyte membrane disposed therebetween. Porous diffusion media (DM) are positioned on both sides of the MEA, facilitating delivery of reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction.
Water must not be allowed to accumulate within the tunnel regions of the fuel cell because of a resulting poor performance of the fuel cell. Water accumulation causes reactant flow maldistribution in individual fuel cell plates and within the fuel cell stack. Additionally, water remaining in a fuel cell after operation may solidify in sub-freezing temperatures, creating difficulties when the fuel cell needs to be restarted. Prior solutions for effectively removing water from a fuel cell have led to increased manufacturing costs and the use of additional components.
Numerous techniques have been employed to remove water from the tunnel regions of the fuel cell. These techniques include pressurized purging, gravity flow, and evaporation. A pressurized gas purge at shutdown may be used to effectively remove water from the tunnel regions of fuel cells. Conversely, this purge increases required shutdown time of the stack and wastes fuel. Positioning of the stack appropriately may allow gravitational forces to remove water from the tunnel regions. Gravitational removal of water may be limited to tunnels having at least a certain diameter. Capillary forces of the conduit and corner wetting by the well known Concus-Finn condition militate against gravitational removal of water. Water removal by evaporation is an insufficient technique as well. Evaporation may require costly heaters and may lead to an undesirable drying of the electrolyte membrane. A dry fuel cell stack militates against proton conduction and prompt starting.
The use of water transport structures and surface coatings are two methods that also allow the tunnel region of a fuel cell plate to transport water into a header region of the fuel cell stack.
Water transport structures, typically in the form of hydrophilic or hydrophobic foam, may be incorporated within the bipolar plate. Water transport structures may be disposed between an active region of the fuel cell and the outlet header. Water transport structures improve removal of liquid water from a fuel cell through a capillary action. While beneficial to the operation and a restart time of the fuel cell, adding water transport structures to the fuel cell stack increases the number of components required to form the bipolar plate. Fabrication and assembly costs of the fuel cell stack subsequently increase when components are added.
Surface coatings may also be used to facilitate a removal of water from the fuel cell. Hydrophobic or hydrophilic surface coatings may be used to increase or decrease the surface contact angle of the bipolar plate, aiding the ability of reactant boundary layer shear forces and pressure to remove water from within the fuel cell. Hydrophobic surface coatings may also be used to militate against a film of water from forming. Coating precursors may be applied to the bipolar plate by spraying, dipping, or brushing, and formed into a hydrophobic or hydrophilic surface coating by secondary operations. Alternately, the coatings may be directly applied. While being less complex and expensive than water transport structures, surface coatings increase the fabrication costs of the bipolar plate.
There is a continuing need for a cost effective fuel cell plate that facilitates a transport of water through the tunnel region of a fuel cell that is inexpensive, minimizes the number of required components, and simplifies plate manufacture.